Holding field to improve the image retention of a cholesteric nematic phase transition liquid crystal imaging system

ABSTRACT

An imaging system including an imaging layer comprising cholesteric liquid crystalline material in an electrical field capable of having field strengths in the cholesteric nematic phase transition field strength range or the Grandjean to focalconic texture transformation field strength range of the liquid crystalline imaging material. The imaging material is typically imaged by placing an imagewise phase-transition or texturetransformation inducing electrical field across a layer of imaging material. The present invention controls such an imaging system by maintaining a substantially uniform holding field across the entire surface of the imaging material, said holding field typically being maintained across the imaging material after it has been imaged. The holding field may be applied in the form of various phenomena. The particular type of holding field applied has field strengths below the cholesteric-nematic phase transition or the Grandjean to focal-conic texture transformation field strength thresholds of the imaging material. The imaging system, and especially its image memory and image relaxation characteristics are controlled by varying the strength of the holding field, and by applying field pulses in imagewise configuration to image the liquid crystalline imaging material, or by pulsing the field uniformly across the entire surface of the imaging material to field strengths above the phasetransition threshold or the texture transformation threshold, thereby enhancing the image already formed in the imaging material.

oo onset-251 United States tatent Wysocki et ai.

l l HOLDING FIELD TOlMPRO-VE THE IMAGE RETENTTON 03' A CHOLESTERIC NEMATIC IHASE TRANSXTKON LIQUID CRYSTAL lMAGiNG SYSTEM Inventors: Joseph J. Wysocki, Webster; James H. Becker, Penfield; Gary A. Dir, Fairport,'all of N.Y.

Xerox Corporation, Stamford, Conn.

Filed: Nov. 29, 197i Appl. No.: 202,746

Related US. Application Data Continuation-impart of Ser. No. 47,698, June l9 1970. abandoned.

[73] Assignee:

(1.5. CI 350/160 LC, 350/150 [51] Int. .Cl. G02! 1/16 References Cited- NITED STATES PATENTS 3/1970 Heilmeicr et a1. 350/160 LC 3/1972 Wysocki et al. 2/1972 Wysocki et al. 350/150 Primary Examiner-Edward S. Bauer Artor'ney, Agent, or F irm-James .1. Ralabate; David C.

Petre; Gaetano D. Maccarone [57] ABSTRACT An imaging system including an imaging layer com- Field of Search... 350/160 LC, 150

prisingcholesteric liquid crystalline'material in an electrical field capable of having field strengths in the cholesteric nematic phase transition field strength range or the Grandjean to focal-conic texture transformation field strength range of the liquid crystalline imaging material. The imaging material is typically im- 'aged by placing an imagewise phase-transition or texture-transformation inducing electrical field across a layer of imaging material. The present invention controls such an imaging system by maintaining a substantially uniform holding field across the entire sur face of the imaging material, said holding field typically being maintained'across the imaging material after it has been imaged. The holding field may be applied in the form of various phenomena. The particular type of holding field applied has field strengths below the cholesteric-nematic phase transition or the Grandjean to focal-conic texture transformation field strength thresholds of the imaging material. The imag- 1 ing system, and especially its image memory and image relaxation characteristics'are controlled by varying the strength of the holding field, and by apply ing field pulses; in imagewise configuration to image theliquid crystalline imaging material, or by pulsing the field uniformly across the entire surface of the imt aging material to field strengths above the phasetransition threshold or the texture transformation threshold, thereby enhancing the image already formed in the imaging material.

22 Claims, 3 Drawing Figures Feb. 5, 1974. i

PAIEHTED FEB FIG. I

. F... m m MW W RI 1H 6 mm mm 0 w n w s m. m m a c c T x w m m E m T. .W V I 2 -1 p I HI M\ q u o R um F L v sW mm P wYsocm JAMES H. BECKER GARY A. 0m-

ATTORNEY I A HOLDING FIELD TO IMPROVE THE IMAGE RETENTION OF ACHOLESTERJC NEMATIC PHASE 'IRANSTTION LIQUID CRYSTAL IMAGING SYSTEM CROSS-REFERENCE TO RELATED CASES This application is a continuation-in-part application of copending patent application Ser. No. 47,698, filed June 19, 1970 now abandoned.

BACKGROUND OF THE INVENTION This invention relates to imaging systems, and more specifically, to an imaging system wherein the imaging member comprises a liquid crystalline material. Furthermore, this invention more specifically relates to the control of such liquid crystalline imaging systems.

Recently there has been substantial interest in the discovery of more useful applications for the class of substances known as liquid crystals." The name "liqties, which are ordinarily associated withliquids. The

optical scattering and transmission characteristics of liquid crystals are similar to those characteristics ordinarily unique to solids. in liquids or fluids, the moleculcs are typically randomly distributed and oriented throughout the mass of the substance. Conversely, in

the crystalline solids the molecules are generally rigidly oriented and arranged'ina specific crystalline structure. Liquid crystals resemble solid crystals in that the molecules of the liquid crystalline substances are regularly oriented in a fashion analogous to but less extensive than the molecular orientation and structure in a crystalline solid. Many substances have been found to exhibit liquid crystalline characteristics in a relatively narrow temperature range; but below that temperature the substances typically appear only as crystalline solids, and above that temperature range they typically appear only as liquids. Liquid crystals are known to ap pear in three different r'nesomorphic forms: the smectic, nc matic,'arid cholesteric. In each of these struc tures the molecules are typically arranged in a unique orientation.

Liquid crystals have been found to be sensitive or responsive to temperature, pressure, foreign chemical compounds, and to electric and magnetic fields, as disclosed in copending application Ser. No. 646,532, filed tem for increasing the speed of response advantageous imaging system wherein the. imaging member comprises a liquid crystalline material, and systems for controlling such'liquid crystalline, imaging members and systems. i i r SUMMARY oFrHE INVENTION It is, therefore, an object of this inventionto provide a novel imaging system. i i

it is another object of this, invention to provide novel liquid crystal imaging system.

lt is another object of this invention to provide a novel system for controlling a liquid crystalline imaging system.

It is another object of this invention to provide a systern for controlling the return of an imaged liquid crystalline imaging member'to its'equilibrium state.

lt is another object of this invention to provide-a sysin re-imaging such a liquid crystalline imaging system,

texture of the liquid crystalline imaging material.

' It is still another object of this. invention to provide v a liquid crystalline imaging system wherein the light permitted .to be transmitted throughthe imaged areas is substantially brighter than that perrnitted'to be trans- Y mitted through such image areasinfprevious liquid crystalline imaging members. r

The foregoing objects and others are accomplished in accordance with this-invention by an imaging system including an imaging layer comprising cholesteric liquid crystalline material in an electrical field capable of having field strengths in thecholesteric-nematic phase transition field strength range or the Grandjeanto focal-conic texture transformation field strength range of the liquid crystalline imaging material. The imaging material is typically imaged by placing an imagewise phase-transition or texture transformation inducing electrical field across a layer of imaging material. The,

June I6, 1967; copending application Ser. No. 4,644,

filed Jan. 21, 1970; Ferguson et al. U.S. Pat.. No. 3,114,838 and 3,410,999; French U.S. Pat. NO.

1,484,584; Fergason U.S. Pat. No. 3,409,404; Water-' i969 now U.S. Pat. No. 3.621224; Ser. No. 85l,708,.

filed Aug. 20, l969 now U.S. Pat. No. 3,622,224; and Ser. No. 367,593, filcd Oct. 20, i969 now U.S. Pat. No.

in new and growing-areas of technology such as liquid crystals, new methods, apparatus, compositions ofpresent invention controls such an imaging system by 5 maintaining a substantially uniform holding field across the entire surface of the imaging material, said holding field typically being maintained across the imaging mai I I terial after it has been imaged. The holding field may be-applied in the form of various phenomena. The particular type of holding field applicdhas field strengths below the cholesteric-nematic phase transition. or the Grandjean to focal-conic texture transformation field strength thresholds of the imaging material. The imaging system, and especially its image memory and image relaxation characteristics are controlled by varying the strength of the holding field, and by applying field pulses in imagewise configuration to imagc the liquid crystalline imaging material, or by pulsing the field uni formly across the entire surface of the imaging material to field strengths above the phase-transition threshold or the texture transformation threshold, thereby en hancing the image already formed in the imaging material. 1 BRIEF DESCRIPTION OF THE DRAWING5 For a better understanding of the invention as well as other objects and further features thereof, reference is matter and articles of manufacture continue to be dis covered for the application of the new technology in a g i new mode. The presentinvention relates to anew. and

It is yet another object of this invention to control the 'f tion with the accompanying drawings thereof, wherein:

FIG. I is a partially schematic, cross-sectional view of a liquid'crystulline imaging member.

FlGQ 2 is a partially schematic, isometric view of an embodiment of a liquid crystalline imaging member 1 .Lwherein the desired-image is definedby the shape of at least one of the electrodes. I FIG. 3 is-a Photornultiplier Response vs. Time plot for a specific liquid crystalline imaging system wherein the substantially?.uniformholding field maintained across the imaging member is varied for various transient responses of the imaging system from the nematic statej'back into the'fcholesteric statc. The data in this figure represents transient responses of the system corresponding to various holding'field strengths, respectively Y f DESCRIPTION .OF THE PREFERRED -awEMBoDlMENTs' in FIG 1 atypical liquid crystalline imaging member 10, sometimes referred to as an electroded imaging sandwich, is shown in partially schematic, cross-section wherein a part of substantially transparent plates 11 having substantially transparent conductive coating 12 upon .the contact surfaces thereof, comprises a parallel pair of substantially' transparent electrodes. An imaging member wherein both electrodes are transparent is prefe'rred where-the imaging-member is to be viewed using transmitted light; however, -a l iquid crystalline imaging member may also be viewed'using reflected light thereby requiring only a single transparent electrode whilefthe other may be opaque. The transparent electrodeis'separated by spacing .member or gasket 13 whichcontains one ormore'voidswhich contain the liquid crystalline film orimagirig layer-which comprises the active element of the imaging member. A field is created between'the electrodes, for example, by an ex- ;Y ternal circuit 15 which" typically comprises a source of potential 16 which is connected across the two electrodesthrough leads 17.; The potential source may be either D.C., A.C., or a combination thereof. In such an imaging system it hasbe'e'n' discovered that when cholesteric liquid crystals or airnixture comprising cholesteric liquid crystallinemat'erials is usedin such an elecs 'trod'e sandwich, the, electrical fields across the liquid crystalline film cause the optical appearance of such.

" liquid crystalline" films to affected areas. I l I V i When high electrical fields arei'placed across the liq- 4 uid crystallineimaging member, an electrical fieldinduced phase transition occurswherein the optically f negative cholesteric liquid crystalline material transfornisinto an optically positive liquidcrystalline state.

visibly change in the field g This transition isb elievedito' be'theresult of the Ch0les-.

I tericfliquidifcrystal{.i material. g transforming into an aligned'nematic liquid crystallinej'mesophase structure.

I. tehi'is-tully described 1 incopeiiding'application Ser. No.

' 1821.565." msdLMayv;5519s now US. Pat. No.

I f- The cholesteric-n'e'matic:phaseltransition is the priused in the present invention] cholesteric liquid crystalline materials [mama cholesteric state are typically 1'; trahslucent; for example, like arnilky white, opalescent layer. This is the way thatsuch'materials frequently ap- This cholestericfnematic phas'transition imaging sys- -3,4552:1 tigthee nimdeems e'j of which'is hereby in- "corporated by reference into'the present specifications j mary liquid 'crystalline imagingl'mechanism which is' pear when first placed in the unbiased electrode sandwich. When a high magnitude electric field is placed across the liquid crystalline film, the field-induced phase transition is observable because the liquid crystalline film becomes transparent in areas where the high electric field is present. When the imaging memher is viewed between polarizers with transmitted light. the areas in which the field-induced phase change has taken place appear dark, while the unchanged, translucent, light scattering, optically active and birefringent. cholesteric areas still retain their white appearance. Although polarizers have just been descn'bed as a convenient means for enhancing the contrast of the imaged areas other enhancement techniques may also be used in place of such polarizers. it is therefore clear that either field or non-field areas in a liquid crystalline imaging member may be used as the desired image, with or without the addition of other means for image enhancement.

The same apparatus and materials used as animaging system wherein the cholesteric-nematic phase transition mechanism is used, may also be used to cause an electrical field-induced texture transition to occur wherein a cholesteric liquid crystalline material initially in its Grandjean-or disturbed" texture is transformed into its focal-conic or undisturbed" texture. The voltages and field strengths typically used for imaging the liquid crystalline imaging members by the texture change mechanism typically have lower magnitudes than the voltages and field strengths used in the phase transition already described above. This electrical field-induced texture transition system is fully described in copending application Ser. No. 867,593.

filed Oct. 20. 1969 now US. Pat. No. 3,642,348, the

reference intothe present specification.

The Grandjean texture is typically characterized by selective dispersion of incident light around a wavelength A, (where A Znp where n equals the index of refraction of the liquid crystalline film and p equals the pitch of the liquid crystalline film) and optical activity for wavelengths of incident light away from A,. If A, is in the visible spectrum, the liquid crystalline. film ap pears to have the color corresponding to A,, and if A, is outside the visible spectrum the film appears colon less and non-scatte'ring. The Grandjean texture of the cholesteric liquid crystalline material is sometimes referred to as the disturbed" texture.

The focal-conic texture is also typically characterized by selective dispersion, but in addition this texture also exhibits diffuse scattering inthe visible spectrum, whether A, is in the visible spectrum or not. The appearance of the focal-conic texture state is typically milky-white when A, is outside the visible spectrum. The focal-conic texture of cholesteric liquid crystals is sometimes referred to as the undisturbed texture.

if the unbiased electrode sandwich containing an imaging composition comprising cholesteric liquid crys pears colored or black. when the electrical field is placed across the liquid crystalline film, with fieldv strengths in the field-induced texture change range. the

texture change to the predominantly focal-conic texture state is observable because the liquid crystalline film becomes white in the field-affected areas when the imaging sandwich is observed'in transmitted or reflected light. The, texture change imaging systemthereby produces white image (or field-affected) areas on a darker colored background. Like the phasetransition system, the texture change imaging system may also be used with or without theusc of polarizers or other image enhancing devices.

in the liquid crystal imaging members of the present invention, the electrodes may be of any suitable trans parent conductive material; however, opaque electrodes may be used, at least in part, in some embodiments. Typical suitable transparent, conductive electrodes include glass or plastic substrates having sub stantially transparent and continuously conductive coatings of conductors such as tin, indium oxide, aluminum, chromium, tin oxide, or any other suitable conductor. These substantially transparent conductive coatings are typically evaporated onto the more insulating, transparent substrate. NESA glass, a tin oxide coated glass manufactured by the Pittsburgh Plate Glass Company. is a commercially available example of a typical transparent, conductive electrode material.-

The spacer, 13 in FlG.'l, which separates the trans parent electrodes and .contains the liquid crystal film between said electrodes, is typically chemically inert, transparent, substantially insulating and has appropri ate dielectric characteristics.

Materials suitable for use as such spacers include cellulose acetate, cellulose triacetate, cellulose acetate crotonate; cholesteryl nonanoate, cholesteryl hexano-- ate; cholesteryl formate; cholesteryl docosonoate; cholesteryl chloroformate; cholesteryl proprionate; cholesteryl acetate; cholesteryl valerate; cholesteryl vacconate; cholesteryl linolate; cholesteryl linolenate; cholesteryl'oleate; cholesteryl erucate; cholesteryl butyrate;

- cholesteryl caprate; cholesteryl laurate; cholesteryl myristate; cholesteryl clupanodonate; ethers of cholesterol such as cholesteryl decyl ether; cholesteryl lauryl ether; cholesteryl oleyl ether; cholesteryl dodecyl ether; carbamates and carbonates of cholesterol such as cholesteryl decyl carbonate; cholesteryl oleyl carbonate; cholesteryl methyl carbonate; cholesteryl ethyl carbonate; cholesteryl butyl carbonate;. cholesteryl docosonyl carbonate; cholesteryl cetyl carbonate; cholesteryl-p-nonylphenyl carbonate; cholesteryl-242- ethoxyethoxy)ethyl carbonate; cholesteryl-Z-(Z-Butoxycthoxylethyl carbonate; cholesteryl-2-(2-methoxye-- thoxylethyl carbonate; cholesteryl heptyl carbamate;

and alkyl amides and alphatic secondary amines derived from 3 B -amino- A 5-cholestene and mixtures thereof; peptides such as poly-benzyl-glutamate; derivatives of beta SIZUSlJfOl such as sitosteryl chloride; and

active amyl ester of cyan henzylidcne amino cinnamate. The alkyl groups in said compounds are iypi fl r saturated or unsaturated fatty acids, or alcohols; havin 3 less than about carbon atoms, andfunsa'turated I chains of less than aboutfive double-bonded olefinic groups. Aryl groups in the above compounds typically comprise simply substituted benzene ring compounds. Any of the above compounds and mixtures thereof may be suitable cholesteric liquid crystalline materials in the advantageous system of the present invention.

Smectic liquid'crystallinematerials are suitable for use as components of the imaging composition in the present invention and such smectic liquid crystal mate rials include: n-propyl-4'-ethoxy biphenyl-4- carboxylate; 5-chloro-6 n-heptyloxy-Z-naphthoic acid; lower temperature mesophases of cholesteryl octanoate, cholesteryl nonanoatc, and other open-chain aliphatic esters of cholesterol'with chain length of 7 or I greater; cholesteryl oleate; sitosteryl oleate; cholesteryldecanoate; cholesteryl laurate; cholesteryl myristatc;

cholesteryl palmitate;cholesteryl stcarate; 4.'-,n-alkoxy' 3 151itrobiphaeyli-carboxylic acids; ethyl p azoxycinnamate; ethyl-p 4 ethofybciizylideneaminocinnantate; ethyl-p-azoxybenzoate; potassiumolcate; ammonium oleate; p-n-octyloxybenzoic acid; the low temperature mesophase of 2-p-n-alkoxy-benzylideneaminofluorenones with chain length of 7 or greater; the low -temperature mesophase of p-(n-heptyl)oxybenzoic acid; anhydrous solid stearate; thallium mixtures thereof andothersu Nematic liquid crystalline materials suitable for use as components of the imaging composition'in the ad vantageous system of the present invention include: p-

azoxyanisole, p-azoxyphenetolc, p-butoxybenzoic acid,

,hexanone,

vacid, butyl-p-anisylidene-paminocinnamate, anisylidene para-aminophenylacetate, p-ethoxy-benzala'mino-a-methyl' cinnamic acid, l,4-bis (p-cthoxy benzylidene) cyclo 4,4'-dihexyloxybenzcne, 4,4' diheptyloxybenzene, anisal-p-amino-azo-bcnzene, ania saldazine, a-benzeneazo- (anisalsa naphthylamine), n,n-nonoxybenzeltoluidine, mixtures of the above and p-methoxy-cinnaminic .many others.

The above lists of material exhibiting various liquid crystalline phases are not intended to be exhaustive or limiting. The lists disclose a variety of representative materials suitable for use in the imaging composition or mixture comprising cholesteric liquid crystalline materials, which comprises the active imaging element in' the advantageous system of the present invention.

The liquid crystalline materials may be prepared by dissolving the liquid crystals or'mixtures thereof in any suitable solvent, for example organic solvents such as chloroform, trichloroethylene, tctrachloroethylene, pe-- troleum ether, methyl-ethyl kctone, and others. The solutidn containing the liquid crystal material isthen typically poured, sprayed or otherwise applied into the imaging members. After cvaporation of the solvent, a thin layer of liquid crystals remains. Alternatively, the individual liquid crystalsof the liquid crystalline mixture can be combined and applied directly by heating the mixed components above the isotropic transition temperature.

I The liquid crystal imaging layers or filmssuitable foruse in the present invention are preferably of a thickness in the range of about 10 mils or less,although thicker filmswill perform satisfactorily in the inventive system. Optimum r esults aretypically. achievedusing (l) stearatc;

7 layers in the thickness range between about A mil and about mils. I t I Although thecholesteri'c-nematic phase transition mechanism and the .Grandjean to focahconic texture transition-mechanism hat-e both been described with chemical effects, staticpressure effects, acoustic and ultrasonic fields; surface forces. and combinations of thesephenomena. I i The use of electrical fieldsto carry out these transitions has been disclosed in the copending patent applications referred to above. The use of magnetic fields alone to effect the cholesteric-nernatic phase transition.

has been disclosed in the/art; See Sackmann et al.. J.

Am. Chem. Soc. 89, 598i (i967).

Thermal fieldscan'be used with mixtures of righthanded and left-handed cholesteric materials or materials with an effective right-handed. or left-handed nature' in a chiral environment to. cause the transitions. lf the materials used in the mixture are suitably chosen the composition can be converted to a nematic (compensated) state by addition or subtraction of thermal energy. is. a change in temperature. At compensation th'enetchirality is zero. Thus a thermal field can cause a cholesteric-riematicor nematic-cholesteric transition. Generally higher resolution can be obtained in transient thermal displays than'with'sta'tic thermal displays; howeveradditional memoryand resolution enhancement can be obtained by appropriate biasing stimuli such as thermal, clectrical ormagnetic'fields. Use of materials having along relaxation time can also provide memory effects. typically of intermediate duration.

. It has been disclosed in the art that ultra-violet radiation can photo-decomposesuitable cholesteric liquid I crystalline materials which'causes a change in pitch (and. therefor the color reflected by) of the resultant material. It has now been found that this phenomenon can be utilized to achieve a cholesteric-nematic phase transition. Furthermore this procedure can provide a permanent or temporary displaydependent upon the nature of the materials an whether recombination of the decomposition products is provided for. In an imaging device a mixture of right-handed and left-handed cholesteric liquid crystalline ingredients which include at least one photo-degradable constituent such as cholesteryl iodide maybe employed. The action of the ultra-violet.radiation:will typically convert the. photode'gradablelmaterial into a different species thereby upsettirig the iriitial ratio of right handed and left-handed ,materials. By afsuitable adjustment of the components of the/mixture and th'e length of exposure. the resultant Qmiittureis typicallyiconvcrted to the nematic state at a Ichos e n.tem'peratureiin the radiation-affected areas whereas the 'non'-imaged or background areas are not converted in this-matter." Ah" interesting feature of j such. an imagingscherne is'that the character of the I image. can be temporarily barrin diffusion) modified changing the'sarnple temperature. Moreover the r mixture ofmatcrials ha ing opposite handcdness may beuchievc'dnotonly by I the initial combination of such g 8 materials but also by the imaging process itself. i.e. by conversion of part of-the material of a given chirality to another of the opposite chirztlity by means of photochemical action.

it is known in the art that static pressure applied to individual cholestcric liquid'crystalline compounds and also mixtures of such compounds, usually reduces the pitch of the material. This effect may be utilized to obtain a nematic-cholcsteric phase transition where a compensated composition is employed. it is to be expected that systems containing mixtures of righthanded and left-handed cholcsteric liquid crystalline materials could be formulated wherein static pressure, rather than reducing the pitch of the mixture, would increase it and affect it enough to induce a nematic transition at a particularoperating temperature. Static pressure would therefore typically change the effective rotary power of the compositions.

A further static pressure effect which would be useful to achieve the phasetransition is based-on the fact that the threshold electric field is a unique feature of any system. Thus for. a given applied voltage, a change in the thickness of the llquid crystalline imaging layer will change the strength of the applied field. When the layer is deformed by static pressure-so that the layer thickness is reduced, the initial applied voltage which was initially not sufficient to cause the phase transition will correspond to a higher field and thus cause the desired cholesteric-nematic phase transition to occur. This mode of operation has particular advantage for a controlled memory display as is disclosed in the present application since the same applied voltage may serve as both the transforming and the holding voltage. This mode of operation typically allows retention of pressure patterns on the liquid crystalline imaging layer long after the pressure itself is removed therefrom. The combination of pressure and voltage can thus be advantageously utilized to sharpen the threshold for the cholesteric-nematic transition and to alter the transformation and relaxation times for this transition.

The foregoing considerations relative to static pres-. sure fields are also pertinent to dynamic pressure fields. Additionally. pressure may give rise to internal voltages (piezoelectric effects) which alter the transitions.

It is known in the art that surfaces may interact with cholesteric liquid crystalline materials principally in establishing the texture of the material. Additionally it is also well known that such surface interactions can be modified by surface treatments for example such as by rubbing the surface or by applying thereto surface coating of materials such as lecithin or the like. Generally speaking, imagewise changes in surface interactions to achieve the phase transition in imagewise fashion may typically be effected.

These various stimuli discussed above have been described with respect to their efficacy in facilitating the cholestericnematic phase transition. These stimuli, in-

dividually or in combination, typically influence the Grandjean-focal-conic texture change as well and therefore the stimuli have utility in texture change display systems as well as in phase transition display systerns.

coatings. here separately designated 12, 20 and 21, upon the contact surfaces of the transparent plates 11. Spacing member'gasket l3 and liquid crystalline film or imaging layer 14 are identical to those described in conjunction with PK]. 1. In FIG. Z-the substantially transparent conductive coating 12 which is behind-the ing sandwich or member, and if said member is observed between crossed polarizers (as image enhancing devices) the imaging layerfrequ ently appears colored liquid crystalline imaging composition 14. is a single,- substantiallycontinuous. electrically conductive, trans-' parent coating. However. transparent continuous conductive coatings 20 and 21, on the substantially transparent plate 11 illustratedin front of the liquid crystalline imaging composition 14 are in complementary imagewise configurations which are separated by an insulating space or material at the boundaries ofthe imagewise configuration electrode designated 21. Because imagewise electrode 21 is not electrically connected to its complementary, coplanar background electrode 20, an appropriately electrically insulated conductive lead 22 is provided to connect the imagewise electrode 21 with the externalcircuitry 15. Although in FIG. 2 the imagewise electrode 21 is illustrated by a more dense dot pattern indicating its substantially transparent and Conductive nature, the complementary background electrode 20, typicallycomprises exactly the same substantially transparent, electrically conductive material which comprises the imagewise electrode 21, and the two areas are illustrated differentlyin FIG. 2 only for the purpose of clarityin distinguishing the. image and background areas of the coplanar, complementary imagewise and background electrodes.

In FIG. 2 the external circuit 15 which is connected to the various electrodes in the imaging member by conductive electrical leads 17, is illustrated containing source of potential [6 and potentiometers 18: and 18b as suitable means fol-controlling the voltage and therefore the fields strengths, in the image and background ar'eas, respectively. in the imaging member. The electrical circuit illustrated in F 16.2 is one suitable means for the application of the desired imaging and holdingfields across the imaging composition, and-it will be understood that any suitable means, such as other electrical circuits for providing the desired fields in the inventive system-may be used in the advantageous system of the present invention. In operation, an imaging composition which is typically in its predominantly Grandjean texture state is typically made to adopt the focal-conic texture by adjusting potentiometers 181' and 18b to establish between electrodes 12 and 20 and electrodes 12 and 21 electric fields within the Grandjean to focalconic texture transformation field strength range of the imaging composition thus causing the liquid crystalline film to appear white or light-scattering. Potentiometer 181 is then adjusted to establish between electrodes 12 and 21 an electric field within the cholesteric-nematic phase transition field strength range of the imaging composition and the field is maintained until the composition between electrodes 12 and 21 becomes clear thus indicating that the phase transition has taken place. The imaging composition between electrodes l2 and 20 remains in a light scattering condition. Hence, an image comprising a clear image area on a light scattering background is formed. A holding field is then up embodiment is illustrative be practiced. I

When the imaging composition is placed in the imagor black, i.e., the imaging composition is initially in its predominantly Grandjean "or disturbed" texture. When an electrical field having field strength in the field-induced texture change range is placed across the liquid crystalline film, for example by adjusting potenti ometer 18:, illustrated in. FIG. 2, the electrical liuld is,

by placing an electrical field having field strength within the field-induced texture change range across such an imaging composition comprising cholestcric liquid crystals. In this way,'the texture of an entire liq,-

uid crystalline-imaging membermay be uniformly maintained in. a substantially uniform electrical field acting upon said' composition.

Where field strengths typically higher than the fieldinduced texture change field strength range, and above the cholesteric-nematic phase transition threshold field strength are used, the imaging composition comprising plied across the entire liquid crystalline film by adjustcomposition. Of course it will be recognized that this :Ored background areas, are produced by the advantacholesteric liquid crystals exhibits the field-induced phase transition transfonning the field-affected areas into the nematic liquid crystalline mesophase structure. This cholesteric-nematic phase transition is the primary imaging mechanism in the present invention whereby the imaging composition is changed from its typically translucent, milky white, opalescent appearance, into a substantially transparent film in the areas where the high electrical fields'are present across the imaging composition. in this way, images, for example images v comprising transparent image areas on white .or colgeous imaging system of the present invention.

In the imaging system and image control system of the present invention it hasbeen found to be particw larly advantageous to maintain a substantially uniform holding field having field strengths below the cholesteric-nematic phase transitionor the texture transformation threshold field strengths of the imaging composb tion comprising cholesteric liquid crystals, and to maintain such a holding field across said composition when- Grandjean to focal-conic texture transformation. Thus the holding field, or stimuli, may. be a magnetic field.

an electrical field, a thermal field various types of pressure fields including static, dynamic and piezoelectric, I

photochemical action tions thereof.

For purposes ofill surface effects and 'combinaand other embodiments may ustrating the invention it will be dB- scribed in detail hereinafter with respe'ctto the'use of p an electrical field as the holding field. By maintaining imaging composition layercan be maintained in a substantially uniform condition, as already described I aboveuFurthermore it has been found that after the imagingcomposition has been imaged by the cholesteric- ,nematic phase transitionmechanism, and the phase x jftratisition; imaging field is removed, that the application of the advantageous holding field of the present invention maintains the composition vin a partially transformed or partially aligned state, which facilitates reimaging of the imaging composition. It is believed that the previously phase-transition imaged areas of the 4 composition aremaintainedin a partially transformed state,.so.that thesubsequent application of a phase-,

transitionimaging field causes the image to re-appear I or to be renewed in a shorter time than was formerlypossible when the imaging composition was allowed to i return to its equilibrium state-in the absence of the advantageous holding field of the present invention. The holding field even makes possible the renewal of an image by briefly-increasing the field strength across the entire surface area of the imaging composition to a ffield strength above-the phase' transition threshold, as well as by the more conventional application of an image'wise phase-transition field. It is therefore seen that the holding. field of the present invention controls the return of the imaged liquid crystalline composition to ,its equilibrium state. The transient return of the composition to its equilibrium stateand the control of that .r transient response of the composition which is made possible by the present invention, may itself be used as an imaging system, as described later herein.

The followingwill be descriptive" of the proposed the oretical mechanismbywhich the process-of the invention is operative. it should be understood. however that affected areas (which may be the image portions or the background portions of the liquid crystal layer) is a relatively unstable one. in these field-affected areas the liquid crystalline material typically desires to return to its original state when the imaging stimulus is removed pronounced when the texture transformation is not the-invention has been 'found to be very effective through"experimentation and any inaccuracy in the theoretical operationthereof as described is not to be construedas bein'g limiting of the-invention. Consider an imaging element comprising a layer of cholesteric liquid crystalline rnaterial between a pair of parallel plates. The imaging processes'typically utilized in the inventive. process, that is the cholesteric texture transformation and {the cholesteric-nematic phase transition, may be. described in their broadest sense as a realignment of the direction of the molecules of the liquid crystalline imaging material with respect to the surface planesof thei plates or the direction of the light impingingot; th Plates. imaging is typically effected by applying a suitable field,e.g. an electric field, across the liquid crystal imaging layer so as: to cause a cholestericnematic' phase transitionor a cholesteric texture trans formation in thefield-affected areas of the liquid crys- "tal layer. .-'Generally,.for each of the various stimuli whichl'can be used to effectimaging, as previously described, there is a threshold field which typically should be exceeded for imaging [to ,occur. These threshold fields are, inter alia, functions of the chemical composition 'ofvthc-liquidj crystalline imaging layer. Typically the threshold fields. are very sharp for the cholestericnernutic phasetransition and relativelyless sharp for the cholesteric texture transformation.

- Tlhus the field-affected areas arein a different state than those areasofthe-liquidicrystal layer across which .to return to its original condition, varies monotonically with the proportion of molecules transformed from their original texture to the other texture when the stimulus is applied thereto. The actual decay times can vary over a relatively wide range and are subject to the thickness of the imaging cell, the liquid crystalline composition, surface treatments, etc. Decay times of from a few seconds to days have been observed.

The present invention is operative by preventing the field-affected areas from returning to their original state for an extended period of time 'after the imaging stimulus has been removed from the liquid crystal layer. This is accomplished by applying to the liquid crystal layer a uniform holding field which is preferably biased closely to the threshold field typically necessary to convert the liquid crystalline material from its origi nal state to its changed state in order for imaging to occur. Generally the duration of the time period over which the image can be maintained according to the invention is related to the sharpness of the threshold field and typically is directly related to the proximity of the holding field to the threshold field.

lt will be recognized by those skilled in the art that when photochemical effects are utilized for imaging the situation is somewhat different than when the other stimuli are used since the changes brought about in theliquid crystalline imaging material by photochemical effects are essentially irreversible. Photochemical effects provide imaging by causing the composition ofthe liquid crystal layer to change in the field-affected areas. it will be. further appreciated that since the threshold fields for the various impinging stimuli are dependent upon the chemical composition of the imaging layer, as aforesaid, the threshold field values for the various stimuli change when the chemical composition of'the liquid crystal layer is altered.

. As illustrated in copending application Ser. No.

821,565 now US. Pat. No. 3,652,148, the cholestericimaging in the advantageous system of the present in-' vention. it should be understood that the phasetransition threshold field strength varies corresponding r the fic rwas not applied. Thecondition of the fielrl- I to the particular imaging composition and other physical environmental conditions at which each imaging composition may be imaged; The phase-transition threshold for. imaging compositions used in the adiantageous control system of the present invention typi cally defines the least upper bound of field strengths suitable forv use as the advantageous holding field in the aging composition comprising cholesteric liquid crystals, of about 60% cholesteryl chloride and about 40% cholesteryl nonanoatc, it will be understood that other.

liquid crystalline systems using other compositions ex-- hibit similar characteristics. The data illustrated in HS. 3 show photomultiplier responses corresponding to the times of such responses, said times being measured from the time that a phase-transition imaged member has the transition-inducing field removed therefrom.

The various response data lines represent thc various.

responses of the particular liquid crystalline imaging system when used with holding fieldshaving field strengths below the phase-transition threshold, and such fields are maintained across the entire surface area of the imaging memberduring various imaging sequences of the imaging cell. In the system representedby the data in FIG. 3 the substantially uniform holding field maintained across the entire surface area of the imaging memberis shown to havebeen varied through the range between about and about 4 "l'voits, with the higher uniform field strengths attenuating the peak transmissivity until the transient response of the imaging cell, here represented by the Photomultiplier Responsevs. Time Data, asymptotically approaches the system'sequilibrium condition, which is here repre. scntedby a photomultiplier response somewhere between about l0 and about mV. The photomultiplier desired phase-transitiori image in the cholesteric liquid crystalline imaging composition; 4 V i in addition, the decreased imaging time (discussed earlier above) can at least in part be explained because the presence of the substantially uniform holding field is believed to maintain the cholesteric liquid crystalline 4 material predominantly in itsfocal-conic or "undisturbed" state thereby eliminating the increment of the response time typically necessary to transform the chi) lesteric liquid crystalline material from its original pre- I dominantly Grandjean or .disturbed"texture stateinto its predominantly undisturbed" state before continu ing through the cholcsteric-nematic phase transition imaging which is-the-primary imaging mechanism in the advantageous system of the present invention.

Chemical compounds which exhibit one or moreof the three well known liquid crystalline mesophases, typically exhibit those liquid crystalline structures only within a corresponding temperature range. In addition, the behavior of such a liquid crystalline compound, or a mixture comprising such compoundsor having cholesteric liquid crystallinecharacteristics, may be differ ent at various temperatures within the liquid crystalline temperature range ofthe particular composition. For

example, the viscosity of such liquid crystalline compo-' sition is typically temperature dependent, and the electrical responses of theliquid crystalline composition, such as those responses which are the operative imaging mechanisms in the advantageous system of the present invention, is dependcntupon the temperature at whichthe novel imaging and ,control system is being operated. For example it might reasonably be expectcd that post-imaging relaxation responses. would take a longer time wherethe liquid crystalline imaging compositionbeing used was at a lower temperature and. therefore typically at a higher viscosity.

tern of the present invention has been described above in conjunction with a simple alpha-numeric display device such as that illustrated in-,FlG. 2, any suitable.

, means for maintaininga holding field having field responses are proportionalto the amount of light transm'itted through the; previously; imaged (phasetransformed) areas of the imagingcompositiomfIhe data in FIG. 3 clearly illustratesthe the maintenance of the substantially uniform electrical holding field, of field strength below the phase-transition threshold field strength, across the entire surface area of such a liquid crystalline. imagingmember, controls the transient re-- sponse of the imaging material in returning to its equi-. librium state after the phase-transition inducing electrical fields, havebcen removed, or reduced. in magnitude. s 7 Another advantageous result from the use of such substantially uniform holding field's maintained across threshold, thereby enabling one to image such a liquid.

strengthsbelow the cholsterio'nematic phase. transition for the particular imagingcomposition, maybe used, along withoth'ermeans for'proyiding im'agewise I fields having strengths; above the, phase-transition crystallineimaging composition by the phase-transition mechanism of the present invention and to control such an imaging system by the advantageoussystemof the present invention, Forfix i plc,anysuitable X -Y ad, dress system may be used, or any other suitable address.

' system, such as an electron beam address systcrnpor the entire surface area of such liquid crystalline imaging members is the resultant reduction in the amount of additional field strength necessary to phase transition image desired image areas of the imaging composition. For example, if, a sulgstantially uniform field strength maintained across the entire surface area of an imaging member is about 50 volts, and the phasetransition threshold field strength forthe imaging memhers is about Ziltlvoits, it is necessary to place only an additional fiel .l ofstrcngth greaterthan about lSi) volts other electrographic techniques such as electric stylii f v or electrographic addresssystcms using-photosensitive I or photoconductiveelements to form electrographic images may be used to electrically address the invem.

tive system... Because of the additional control provided over the phase transition imaging mechanism by the instant ine e imaging un... m 2 2.- t t ys m th o .the use ofa substantially uniform holdingficld'offield strength below the phase-transitionthreshold. field strength, it .is also possible to rcnewa ,phase transformed image by uniformly pulsingthe field across I the entiresurl'uce arcaof the imaging composition to a field strength of Qmagnitudo above the phases transition threshold. for ashort time; ,Ll ereby giving tlie Although-theadvantageousimagingand control sys I l the phase-transition field strengths, while not maintain- Instill another mode, the advantageous system of the present invention maybe used to control the amount of light transmitted. through the previously. imaged 1' (phase-transformed) areas of an imaging composition comprising cholesteric liquid crystals by using the short brightburst of lighttran'smitted through such areas,

when such' areasexhibit their maximum transmissivity as a. unique transient imaging system in itself. As al I "ready noted above, these-periods of maximum transmissivity of the previously imaged areas of the imaging composition-correspond to the peak photomultiplier respori'sesyas illustrated for example in FIG. 3. By con- I trolling the uniformly maintained field having field strengths below the phase-transition threshold, as described in the present invention, the length and timing "of theperiodof'r'naxirnum transmissivity ofthe imaging- "icom p'osition may be controlledto' providefor an enhancedj contrast display which may operate between the transformedtnematic) state and the transient state through'which the composition passes, and especially I Tthe period of maximum transmissivity, while resuming its equilibrium state. Such an'enhanced contrast imaging system istypically' viewedwhen the material is in thesta'te correspondingito the rnaximum transrnissivity Y andthe tim-ing of the mai'tirnum'transmissivity may be synchrt'anized with'a transmittedflash of light thereby further enhancing the contrast displayed by the imaging cell during thebrief transient period of maximum transmissivity. In still a further application of this novel imaging system, the imaging member may be repeatedly cycled through the condition of maximum trans missivity'with such frequency that an image of substantially'constantbrightness isproduced in the imaging 1' member, this brightness corresonding to the maximum transmissiv'ity condition of the imaging member.

ln'add'itio'n to the primary. imaging mechanism (the phase-transition imaging system already described in detail herein), imaging compositions comprising cholesteric liquid crystalline'm'aterials may be imaged by the texture-transition; jsystern" (already described herein) by. placing ima'gewise fields having field ifstrengths abovelthe texture transition threshold of'said I composition across the desired image areas of the imaging composition in a fashion analogous to the rnan- "nerlin which" theTIphase-tran'sition imaging system is "controlled by theadvlantageous holding voltages of the present invention; holding voltages having field strengths below the texturechan'ge transition threshold of theimaging. composition control the transient relaxation o a't'exturefchangeimaged composition from its predominantly.fundisturbedljtexture state back into 1 its predominantly .Grandje'ari'or disturbed" texture state; ln this imaging controlsystem, the holding voltage i's-believed tomaintain the previously texture changed image areas of the composition in a more texture transforrned. state than the original, unimaged,

' previously imaged areas sufficient time to respond to EXAMPLE I 1 An imaging member is provided having a pair of parallel plate transparent electrodes each comprising a substantially transparent continuous coating of tin oxide on a glass substrate, and between the transparent coatings, a spacer gasket of Mylar film, a polyester resin film available from Dup'ont enclosing an about 3 8 micron thick film of imaging composition here comprising a mixture of about 60% cholesteryl chloride and about 40% cholesteryl nonanoate. This imaging member is placed between polars, a polarizeranalyzer com- -bination on each side of imaging member, and an. in-

candescent light source is placed on the outside of the polarizer on the polarizer side of the imaging member and light source, polarizer, imaging member and analyzer, are aligned with a phototube detector which is out side the analyzer on the analyzer side of the imaging member. The imaging member is provided with suitable electrical circuitry to provide an electrical field between the electrodes, and included in said circuitry is a switch for applying voltage to or removing it fromthe imaging member and simultaneously activating the sweep of an oscilloscope which meters the phototube analyzer combination; the imaging member described response. The source-polarizer-imagingmember analyzer-phototube apparatus is: A Leitz Orth'olux polarizing microscope, equipped with a hot stabe and a l0-power objective, which serves as the polarizerabove placed onthe hot. stage; the phototube an RCA 7102 photomultiplier with an 8-1 spectral response,

placed in optical alignment with the other elements of the systermand. an incandescent light bulb.

The cholesteric-nematic phase transition threshold voltage for this composition is about 72 volts at about 25C. This cell is typically imaged at about room temperature. A voltage of about 400 volts is applied to the imaging member or cell, and the imaging composition transforms-to the nematic state in about 0.15 seconds.

The transformed material appears dark when the imaged member isobserved between polars in transmitted light. The field is removed by shorting the leads to the imaging member and the liquid crystalline composition relaxes back into the cholesteric state in a transient response which includes a time period of substantially increased light transmissivity. This maximum transmissivity occurs at about 4 seconds after shorting the imaging member, and the material continues to gradually as predominantly Grandjean'or disturbed" texture. state sume the cholesteric state, and first, the predominantly focal-conic texture. An equilibrium is reachedin about 16 to 20 seconds and upon further relaxation the sample will slowly return ,to its predominantly Grandjean texture. This latter and final relaxation may take quite a long time, ranging from minutes to hours.

A voltage of about 400 volts, (i.ev above the phasetransition threshold field strength) is reapplied, and the imaging cycle repeats itself. The material transforms from its predominantly Grandjean texture to its predominantly focal-conic texture before the cholestericmum transmissivity in the transient response is now ob- EXAMPLE ll The imaging member of Example I is provided with an imaging composition comprising about 60% anisyli-. dene-p-n-butyl aniline and about 40% cholesteryl chloride. This composition has a shorter natural relaxation time than the composition used as the imaging member in Example I. This imaging member is imaged as described as in Example I and is then shorted, without the inventive holding voltage, and the maximum transmissivity occurs about 6 milliseconds after shorting when operated at about 26C. The cholestericnematic phase transmission imaging is achieved by applying about 300 volts to the imaging member, which is subsequently reduced to a holding field voltage of about 1 10 volts, and the maximum transmissivity in relaxation is observed to occur at about 18 milliseconds after reduction of the imaging field to the holding field strength.

EXAMPLE 1]] An imaging member like the one described in Example l is provided with the additional feature that one of the transparent electrodes comprises complementary, coplanar image and background configurations which are separated by an insulating space at the boundaries of the image configuration, as described in H6. 2-. This imaging member is imaged by the methods described in Example I by placing a voltage of about 400 volts between the imagewise electrode and the electrode on the opposite side of. the film of imaging composition, thereby imaging the film as described in Example I.

After phase-transition imaging, the imagewise phasetransition field is removed, and a holding voltage of about -30 volts is placed across the entire surface area of the imaging film, i.e. across both the image and background electrodes. The maximum transmissivity in the imaged areas is observed to occur about l3 seconds after removing the phase-transition imaging field. The phase-transition voltage. of about 400 volts, is then again placed across the film using only the imagewise electrode. and the imaged areas are observed to reimage inn shorter time than was required for the initial imaging of the film.

EXAMPLE W J An imaging member using imaging composition and spacer gaskets similar to the ones described in Example l is provided wherein the substantially transparent elec trodes comprise glass substrates upon which separate strips of substantially transparent conductive tin oxide has been coated, and the substantially transparent elec trodes are oriented so that the conductive strips on the respective electrodes cross each other in an .r-y matrix or grid pattern with the imaging film between the two sets of conductive strips. Each conductive strip and each set of parallel strips is electrically connected to electrical circuitry suitable for selective operation of each strip individually. The second strips in each of the respective sets or electrodes are connected to opposite poles of a source of potential difference toproduce an electrical field of about 400 volts across the imaging film thereby causing phase-transition imaging in the area of the liquid crystalline film between the intersection of the two electrically connected strips, which areon opposite sides of the film. After phase-transition imaging in this area the second strips are disconnected, and the third strips; respectively, on each electrode are electrically connected to the 400 volt phase-transition field, and the area between the intersection of the third strips is phaseftransition imaged. The third strips are similarly disconnected and the fourth strips are connected thereby imaging the area of the film between the intersection of the fourth strips. In this way a diagonal line of rectangular imaged areas is created in the imaging film. However, the first imaged rectangular area begins the transient ncmatic-cholestcric relaxation upon removal of the phase-transition field, and this image area may need periodic renewal as the other areas of the desired image are imaged by the phasetransition mechanism. Such previously imaged areas are renewed by electrically connecting all of the strips on both sides of the imaging film to the phase-transition voltage of about 400 volts, for a few milliseconds,

whereby the previously imaged areas are renewed by the phase-transition mechanism-but the previously unimaged areas are not given sufficient time under the infield to exhibit the phase- I fluence of a phase-transition transition.

' EXAMPLE v The imaging member discribed in Example I is provided and placed in a magnctic'field' having a field I strength of about 5000 gauss, with the direction of the magnetic field being parallel to the direction in which the imaging member is viewed. An imagewise electric 1 field of about 2 X [0 volts/cm is then applied across the liquid crystalline imaging composition and are con nected thereby imaging the area of the filmbetween the intersection of the fourth strips. in this way a diago-- nal line of rectangular imaged areas is'created in the imaging film. However, the first imaged rectangular area begins the transient nematic-cholesteric relaxation upon removal of the phase-transition field, and this image area may need periodic renewal as the other areas of the desired image are imaged by the phasetransition mechanism. Such previously imaged areas are renewed by electricallyconnecting all of the strips on both sides of the imaging film to the phase-transition voltage of about 400 volts, for a few milliseconds.-

whereby the previously imaged areas are renewed by the phase-transition mechanism, but the previously unimaged areas are not given sufficient time under the in- .-fluence of a phase-transition field to exhibit-the phasetransition.

- t EXAMPLE v The imaging member described in Example I is provided and placed in a magnetic field having a field strength of about 5000 gauss, with the direction of the magnetic field being parallel to the direction in which the imaging member is viewed. An imagewise electric field of about 2 X l" volts/cm is then applied across the liquid crystalline imaging composition and an image is observed. Theelectric field is then removed from the imaging member and the magnetic field provides a holding field.

EXAMPLE Vl An imaging member such as that'described in Example l is provided'with the exception that the surfaces of the conducting plates are given a surface treatment by rubbing them with lecithin. An imagewise thermal sig- 1 nal is'directed upon the imaging member. the signal being sufficient toraisc the temperature of the liquid crystalline imaging" composition to about42C in the imagedareas. At this temperature the imaging composition 'becomenematimSurface interaction causes the mo'lecu1es;to align} perp'endicular to the surface of the imaging member parallel to the viewing direction.

I f'fexamrta vn I A layer ofa compositionimade up of 59 percent cholesteryl chloride and 4lpercentcholesteryl nonanoate is placed betweenja pair of,parallel plates such as are described in Example l'separated by a 1 mil thick Mylar spacer. The cholesteric nematic phase transition threshold voltage ,forthis composition is about 35 volts at about 30C. The sample is thermally biased to about 30C and a holding voltage of about l5 volts is applied. A thermal image signal is applied to the sample raising the temperature of the imaging: composition to about 40C in the imagedareas. The cholesteric-nematic "EXAMPLE Vlll Y The imaging and liq'uid crystalline composition I described in ExampleVIl are again used. The cell is imagedat about 26C. An imagewise electrical field of I about 400 volts is applied imagewise across the liquid crystalline layer anda cholestericmematicphase transitionoccurs in the field-affected areas of the imaging layer; The temperature of the sample is raised to about 40C and thefield removed by shorting the leads to the imagingmember. The image is' retained for about seconds whereas it is only retained for about 4 seconds 'without the thermal holding field,

' EXAMPLE lX Theimagin'g cell and liquid crystalline composition is biased to a temperature of about 38C. A thermal image is applied to the imaging cell raising the temperature of the liquid crystalline composition to about 42C in the field-affected areas and the imaging composition transfonns to the nematic state. The thermal field is removed and the temperature of the sample returns to about 38C. The image is retained for about 10 seconds whereas it is only retained for about 4 seconds at C.

EXAMPLE X The imaging cell and liquid crystalline material described in Example Vll are again used. The sample is biased to a temperature ofabout 59C. The cholestericnematic phase transition threshold voltage at about 59C is about 86 volts. A thermal image-wisefield is applied to decrease the temperature of the sample in the field-affected areas to about 42C and the imaging composition transforms to the nematic state. The thermal image is removed and the temperature of the sam ple returned to about 59C. At the same time an electric field of about 60 volts is applied across the sample.

This holding field'allows the image to be retained for about 4 seconds whereas it is only retained for about 0.2 seconds without the holding voltage.

EXAMPLE XI 15 microns. The imaging cell is subjectedto an image- 7 wise magnetic field having a field strength of about 10 kilogauss with the direction of the magnetic field being perpendicular to the surface plane of the imaging cell l described in Example Vll areagain used. The sample and parallel to the viewing direction. An imagewise phase transition occurs. The strength of the magnetic field is then reduced to about 5 kilogauss which is sufficient to provide a holding field.

EXAMPLE X A layer of an imaging composition made up of about 4 position has a pitch of about 13 microns. The imaging cell is placed in an imagewise magnetic field having a field strength of about 8.5 kilogauss with the direction of the magnetic field being perpendicular to the surface plane of the imaging cell and parallel to the viewing direction. An imagewise phase transition occurs. The strength of the magnetic field is reduced to about 5 kilogauss which is sufficient to provide a holding field.

Although specific components and proportions have been stated in the above description of the preferred embodiments of the advantageous liquid crystalline imaging system and imaging control system of the present invention, other suitable materials and variations of the various steps in the system as listed herein, may be used with satisfactory results and various degrees of quality. In addition, other materials and steps may be added to those used herein and variations maybe made in the process to synurgize, enhance or otherwise modify the properties of the invention. For example, various other mixtures of liquid crystals which will undergo the phase-transition and texture transition imaging processes may be discovered and used in the system of the present invention, and such mixtures may require somewhat different thicknesses, threshold fields, holding fields, temperature ranges, and other imaging conditions for preferred results in accordance with the present invention. Likewise, various other means of creating electrical fields or altering the threshold voltages, and other means of addressing the inventive imag- 9. The method of claim 3 wherein the layer of imaging composition is viewed with means for enhancing contrast between the image and backgroundareas.

ing members may he used with satisfactory results in the present invention.

lt will'be understood that various changes in the details, materials, steps, and arrangements of elements which have been herein described and illustrated in order to explain the nature of the invention. will occur to and may be made by those skilled in the art, upon a reading of this disclosure and such changes are intended to beincluded within the principle and-scope of this invention.

What is claimed is:

1. An imaging method comprising providing a layer of imaging composition comprising cholesteric liquid crystalline material at a temperature in the cholestericnematic phase transition temperature range of said layer comprising image portions comprising the phasetransformed nematic liquid crystalline state and complementary background portions comprising the cholesteric liquid crystalline state, and

applying a holding field across the entire area of said layer, said field having a field strength below the phase transition field strength of the imaging composition and at least about 50 percent of the phase transition field strength.

2. The method of claim 1 wherein said holding field comprises an electric field.

3. The method of claim 2 wherein the imaged of imaging composition is provided by providing a layer of imaging composition comprising cholesteric liquid crystalline material in the cholesteric-nematic phase transition temperature range of said composition, and

applying an imagewise electrical field across said composition, said imagewise field of a field strength within the cholesteric-nematic nematic phase transition electrical field strength range of said composition.

4. The method of claim 3 wherein said electrical holding field is maintained across the entire surface area of said layer throughout the relaxation of the imlayer aged areas from the imaged nematic state back into the cholesteric state, whereby the relaxation response of the composition is controlled.

5. The method of claim 3. wherein the imaging composition comprises a mixture of cholesteric and nematic liquid crystalline materials.

6. The method of claim 3 wherein the imaging composition comprises a mixture of cholesteric and smectic liquid crystalline materials.

7, The method of claim 3 wherein the layer of imaging composition is of a thickness not greater than about l0 mils.

8. The method of claim 7 wherein the layer of imag-.

ingcomposition is of a thickness in the range between about V4 mil and about 5 mils.

10. The method of claim 9 wherein the imaging composition is viewed between polarizers.

H. An imaging method comprising the method of claim 3 and thereafter re-imaging the layer of imaging composition by applying an imagewise electrical field across said composition, said imagewise field of a field strength within the cholesteric-nematic phase transi tion electrical field strength range of said composition.

12. A method of renewing an image comprising the method of claim 3 and thereafter applying an electrical field across the entire surface area ofsaid layer ofirnaging composition, said field having a field strength within the cholesteric-nematic phase transition electrical field strength range of said composition, and maintaining this field across the entire surface area of said layer for a length of time wherein the previously imagewise cholesteric-nematic phase transformed areas have the phase transformation renewed therein, but the background areas remain substantially in the cholesteric liquid crystalline state.

13. An imaging method comprising a. the method of claim 3 wherein said electrical holding field is maintained across the entire surface area of said layer until previously imaged areas of said layer are about at the point of maximum transmissivity in thenematic-cholesteric relaxation, and I area of said layer until previously imaged areas of said layer are at about the point of maximum transmissivity in the nematic-cholesteric relaxation, and

b. said layer is then re-imaged by applying an clectri cal field across the entire surface area of said layer of imaging composition, said field having a field strength within the cholesteric-nematicphase transition'clectrical field strength range of said composition, and maintaining this field across the entire surface area of said layer for a length of time wherein the previously imagewise cholcstericnematic phase transformed areas have the phase transformation renewed therein, but the background areas remain substantially in the cholesteric liquid crystalline state.

15. An imaging method comprising cyclicly repeating steps (a) and (b) of claim 14 a plurality of times whereby an image is maintained in the layer of imaging composition.

16.Animaging method comprising providing an imaging member comprising a layer of imaging composition comprising cholesteric liquid crystalline material, said layer between a pair of electrodes, at least one of which is substantially transparent and at least one of which is shaped in a desired image configuration, and means for applying an electrical field between said electrodes across the layer of imaging composition,

and performing the method of claim 3 upon said imaging composition.

17. The method (if claim 16 wherein both electrodes 'are shaped in a desired image configuration.

' 18. The method of claim 17 wherein the electrodes comprise .a pair of'sets of parallel strip electrodes, and said pair of parallel strip electrodes are oriented crossing eachother in an X-Y grid pattern.

19. The method of claim 16 wherein both electrodes are substantially transparent.

20. The method of claim 16 wherein the layer of imaging composition is of a thickness not greater than 

2. The method of claim 1 wherein said holding field comprises an electric field.
 3. The method of claim 2 wherein the imaged layer of imaging composition is provided by providing a layer of imaging composition comprising cholesteric liquid crystalline material in the cholesteric-nematic phase transition temperature range of said composition, and applying an imagewise electrical field across said composition, said imagewise field of a field strength within the cholesteric-nematic nematic phase transition electrical field strength range of said composition.
 4. The method of claim 3 wherein said electrical holding field is maintained across the entire surface area of said layer throughout the relaxation of the imaged areas from the imaged nematic state back into the cholesteric state, whereby the relaxation response of the composition is controlled.
 5. The method of claim 3 wherein the imaging composition comprises a mixture of cholesteric and nematic liquid crystalline materials.
 6. The method of claim 3 wherein the imaging composition comprises a mixture of cholesteric and smectic liquid crystalline materials.
 7. The method of claim 3 wherein the layer of imaging composition is of a thickness not greater than about 10 mils.
 8. The method of claim 7 wherein the layer of imaging composition is of a thickness in the range between about 1/4 mil and about 5 mils.
 9. The method of claim 3 wherein the layer of imaging composition is viewed with means for enhancing contrast between the image and background areas.
 10. The method of claim 9 wherein the imaging composition is viewed between polarizers.
 11. An imaging method comprising the method of claim 3 and thereafter re-imaging the layer of imaging composition by applying an imagewise electrical field across said composition, said imagewise field of a field strength within the cholesteric-nemaTic phase transition electrical field strength range of said composition.
 12. A method of renewing an image comprising the method of claim 3 and thereafter applying an electrical field across the entire surface area of said layer of imaging composition, said field having a field strength within the cholesteric-nematic phase transition electrical field strength range of said composition, and maintaining this field across the entire surface area of said layer for a length of time wherein the previously imagewise cholesteric-nematic phase transformed areas have the phase transformation renewed therein, but the background areas remain substantially in the cholesteric liquid crystalline state.
 13. An imaging method comprising a. the method of claim 3 wherein said electrical holding field is maintained across the entire surface area of said layer until previously imaged areas of said layer are about at the point of maximum transmissivity in the nematic-cholesteric relaxation, and b. said layer is then re-imaged by reapplying the same imagewise electrical field across said composition, said imagewise field of a field strength within the cholesteric-nematic phase transition electrical field strength range of said composition.
 14. An imaging method comprising a. the method of claim 3 wherein said electrical holding field is maintained across the entire surface area of said layer until previously imaged areas of said layer are at about the point of maximum transmissivity in the nematic-cholesteric relaxation, and b. said layer is then re-imaged by applying an electrical field across the entire surface area of said layer of imaging composition, said field having a field strength within the cholesteric-nematic phase transition electrical field strength range of said composition, and maintaining this field across the entire surface area of said layer for a length of time wherein the previously imagewise cholesteric-nematic phase transformed areas have the phase transformation renewed therein, but the background areas remain substantially in the cholesteric liquid crystalline state.
 15. An imaging method comprising cyclicly repeating steps (a) and (b) of claim 14 a plurality of times whereby an image is maintained in the layer of imaging composition.
 16. An imaging method comprising providing an imaging member comprising a layer of imaging composition comprising cholesteric liquid crystalline material, said layer between a pair of electrodes, at least one of which is substantially transparent and at least one of which is shaped in a desired image configuration, and means for applying an electrical field between said electrodes across the layer of imaging composition, and performing the method of claim 3 upon said imaging composition.
 17. The method of claim 16 wherein both electrodes are shaped in a desired image configuration.
 18. The method of claim 17 wherein the electrodes comprise a pair of sets of parallel strip electrodes, and said pair of parallel strip electrodes are oriented crossing each other in an X-Y grid pattern.
 19. The method of claim 16 wherein both electrodes are substantially transparent.
 20. The method of claim 16 wherein the layer of imaging composition is of a thickness not greater than about 10 mils.
 21. The method of claim 16 wherein the imaging member is viewed with means for enhancing contrast between the image and background areas.
 22. The method of claim 21 wherein the imaging member is viewed between polarizers. 